If you're shooting for a 5-5.25 gallon batch and you collected 8 gallons preboil and your boil time is only 60 minutes then that would account for your loss of efficiency. I can't see you losing 2.5+ gallons to evaporation in 1 hour. It's most definitely not ruined. Simply boil until you reach your correct starting gravity, regardless of volume (might take an addition 30-60 minutes). Your only hop additions are 60 minutes (bittering) so you might get a couple extra IBUs from them by boiling longer but probably not even noticable; in other words, no late hop additions to ruin by boiling longer.
BTW, what was your brix reading?